Transistors, as is well known in the art, are the building blocks of all integrated circuits. Modern integrated circuits interconnect literally millions of densely configured transistors that perform a wide variety of functions. To achieve such a dramatic increase in the density of circuit components has required microelectronic manufacturers to scale down the physical dimensions of the transistor below the sub-micron regime. One common type of transistor used in a sub-micron microelectronic device utilizes a polysilicon gate electrode. However, polysilicon gate electrodes may suffer device performance degradation due to the polysilicon depletion effect, wherein an electric field applied to a polysilicon gate sweeps away carriers (holes in a P-type doped polysilicon, or electrons in an N-type doped polysilicon) so as to create a depletion of carriers in the area of the polysilicon gate near the underlying gate dielectric of the transistor. This depletion effect results in a reduction in the strength of the electric field at the surface of the microelectronic when a voltage is applied to the polysilicon gate electrode, which can have an adverse affect on the transistor performance.
One way of improving the performance of sub-micron microelectronic transistors is to use metal gate electrode technology. While replacing traditional polysilicon gate electrodes with metal or metal alloy gate electrodes eliminates the polysilicon depletion effect, there are problems associated with the use of such metal gates. One problem encountered is that the carriers from the metal gate can diffuse into the underlying gate dielectric material, thus causing shorting of the microelectronic device.
Another problem encountered with the use of metal gates is work function mismatch, wherein the work functions of the metal gate p and n-channel transistors do not match the work functions of the p and n channel transistors of the polysilicon gate. It is well-known in the art that in CMOS (complementary metal oxide semiconductor) devices, there are generally two different types of gate electrodes, an n-channel gate electrode and a p-channel gate electrode, which have two different work function values (i.e. an energy level of a semiconductor which can be near the valence or the conduction band of the material). The work function values are about 4.2 and 5.2 electron volts for the n and p-channel electrodes respectively, and they are generally formed by doping the polysilicon to be either n or p type. In contrast, previously proposed metal gate electrodes have focused on using one type of metal for both channels of the gate electrode, with a work function that is located in the middle of the p and n channel work function range (e.g. about 4.7 electron volts). A drawback to this mid-range work function approach is that this type of metal gate device cannot easily achieve a desirable small threshold voltage, which is the amount of voltage that determines the transistor's on and off states, without causing degradation in device performance.
Accordingly, what is needed is a structure and method for obtaining the desired work function values for a metal gate transistor device, which eliminates the problems of polysilicon depletion and that may also eliminate metal diffusion into the gate dielectric layer.